1. Field of the Invention
The present invention relates to a valve drive control device in which an intake valve or an exhaust valve of a valve actuating line of an engine is driven by a cam lobe removably engaged with a cam shaft.
2. Description of the Background Art
Such a valve drive control device is exemplified in the prior art by Japanese Patent Laid-Open No. 150016/1987 or 164509/1991, for example. In such a device, the cam shaft of the engine valve actuating line is equipped with a joint pin which can come into and out of the cam shaft from the outer circumference of the same. A rotatable cam lobe fitted in the cam shaft is formed in its inner circumference with a joint hole, into and out of which the joint pin can go. When the joint pin is protruded to enter the joint hole in the inner circumference of the cam lobe, this cam lobe rotates together with the cam shaft to actuate the valve. On the other hand, when the joint pin is extracted from the joint hole, the cam lobe is set free for rotation relative to the cam shaft thereby deactivating the valve.
Immediately thereafter, however, the joint pin comes out of the joint hole to set the cam lobe free for rotation. This cam lobe has a rotational inertial force and may actuate the valve. In the action of the valve at this time for the free rotation, the lift curve is not regulated by the cam shape so that the valve is abruptly seated thereby augmenting the noise and lowering the output.
As disclosed in Japanese Patent 197613/1987, therefore, there is an example of a structure for stopping the cam lobe which is set free for rotation.
In this example, a cylindrical portion integrated with the cam is rotatably interposed between the cam shaft and a support member and is formed with a diametric through hole. The cam shaft and the support member are individually formed with retaining holes which can be opposed to the through hole. A lock plunger (or the joint pin), which is movable in the through hole of the cam cylindrical portion, moves back and forth in the retaining hole of one of the cam shaft and the support member thereby retains one of them.
When the lock plunger to be hydraulically driven is retained by the retaining hole of the cam shaft, the cam rotates together with the cam shaft to activate the valve. On the other hand, when the lock plunger is disengaged from the cam shaft and retained in the retaining hole of the support member, the cam is set free for rotation and comes into engagement with the support member to stop the rotation.
There is another example (as disclosed in Japanese Patent Laid-Open No. 105216/1988), in which the cam is allowed to slide only in the axial direction relative to the cam shaft but is made to rotate together with the cam shaft at all times. A bucket (or the valve lifter) is formed in its portion with a relief so that the cam lobe may be caused to pass around the relief of the valve lifter as the cam slides, to thereby inactivate the valve.
In the case of the former construction (i.e., Japanese Patent Laid-Open No. 197613/1987), however, the lock plunger goes, before being retained, into the retaining hole of one of the cam shafts or the support member. As a result, the protruding timing is restricted to a limited short time period of the cam rotation phase. If poorly timed, the lock plunger fails to completely go into the retaining hole while leaving its portion unretained thereby adversely affecting the valve.
In the latter case (i.e., Japanese Patent Laid-Open No. 105216/1988), on the other hand, the relief formed in the valve lifter is always fixed in a predetermined position but should not shift. As a result, the valve lifter has to be equipped with a rotation preventing structure so that the structure is complicated. This can adversely raise the parts production cost.